This is a pre peer-review version and may differ from the published version in Nature Astronomy. The final authenticated version is available online at https://www.nature.com/articles/s41550-021-01458-1.

Quiescent Ultra-diffuse in the field originating from backsplash orbits

Jose´ A. Benavides1,2,* Laura V. Sales3, Mario. G. Abadi1,2, Annalisa Pillepich4, Dylan Nelson5, Federico Marinacci6, Michael Cooper7, Ruediger Pakmor8, Paul Torrey9, Mark Vogelsberger10 and Lars Hernquist11 .

1Instituto de Astronom´ıa Teorica´ y Experimental, CONICET-UNC, Laprida hand, for the few quenched UDGs that have been discovered in the 854, X5000BGR, Cordoba,´ Argentina. 2Observatorio Astronomico´ de field, the mechanism responsible for removing the gas and halting Cordoba,´ Universidad Nacional de Codoba,´ Laprida 854, X5000BGR, star-formation remains unknown. On the theory side, progress requires Cordoba,´ Argentina. 3Department of Physics and Astronomy, University of high resolution cosmological simulations that are able to resolve the California, Riverside, CA, 92521, USA. 4Max-Planck-Institut fur¨ Astronomie, myriad of environments and physics involved in this problem; from the Konigstuhl¨ 17, 69117 Heidelberg, Germany. 5Institut fur¨ theoretische Astro- formation of isolated dwarfs in their haloes, to their interactions with physik, Zentrum fur¨ Astronomie, Universitat¨ Heidelberg, D-69120 Heidelberg, filaments, groups and clusters. Such simulations have only recently Baden-Wurttemburg,¨ Germany. 6Department of Physics and Astronomy become possible, with the TNG50 simulation – used here – among ”Augusto Righi”, University of Bologna, I-40129 Bologna, Italy. 7Department those with the highest resolution available 25, 26. of Physics and Astronomy, University of California, Irvine, 4129 Frederick Reines Hall, Irvine, CA 92697, USA. 8Max-Planck-Institut fur¨ Astrophysik, We use the stellar mass–size relation defined by all simulated Karl-Schwarzschild-Str. 1, D-85748 Garching, Germany. 9Department galaxies (Fig. 1, grey dots) to define our sample of field UDGs as of Astronomy, University of Florida, 211 Bryant Space Science Center, those central galaxies (excluding satellites) with stellar mass in the 10 Gainesville, FL 32611, USA. Department of Physics, Massachusetts dwarf range (log(M?/M ) = [7.5,9], shaded pink region) and a stellar Institute of Technology, Cambridge, MA 02139, USA. 11Institute for Theory size above the 95th percentile at a given mass (magenta stars). Our and Computation, Harvard-Smithsonian Center for Astrophysics, Cambridge, definition overlaps with observational samples of UDGs14, 18, 27, 28. We MA 02138, USA. study the origin of the extended sizes of these galaxies elsewhere (J.A.B. et al., manuscript in preparation) but a brief summary is given Ultra-diffuse galaxies (UDGs) are the lowest-surface bright- in Fig. 6 in the Supplementary Information. ness galaxies known, with typical stellar masses of dwarf galaxies but sizes similar to larger galaxies like the Milky Way1. The colours (g-r) of our simulated UDGs in Fig. 2 show a clear reason for their extended sizes is debated, with suggested internal bimodality: most central UDGs are in the ‘blue cloud’, suggesting processes like angular momentum 2, feedback 3, 4 or mergers 5 young stellar populations as expected for dwarfs in the field, while versus external mechanisms 6–9 or a combination of both 10. Ob- 23.7% of our simulated UDGs are along the red sequence. The mass servationally, we know that UDGs are red and quiescent in groups distribution is non-uniform, with red UDGs being more common and clusters 11, 12 while their counterparts in the field are blue and towards the lower masses, where also, at the same mass, field UDGs star-forming 13–16. This dichotomy suggests environmental effects have a higher fraction of red objects than normal dwarfs in the field as main culprit. However, this scenario is challenged by recent (See upper panel of Fig. 6 in the Supplementary Information). The observations of isolated quiescent UDGs in the field 17–19. Here we inset panel shows that their colours correlate with their star formation use ΛCDM (or Λ cold , where Λ is the cosmological rates, with the blue UDGs occupying the “main sequence” of star- constant) cosmological hydrodynamical simulation to show that forming galaxies and the red UDGs showing negligible star formation isolated quenched UDGs are formed as backsplash galaxies that today. were once satellites of another galactic, group or cluster halo but are today a few Mpc away from them. These interactions, albeit A close inspection of the histories of our red quiescent UDGs arXiv:2109.01677v1 [astro-ph.GA] 3 Sep 2021 brief, remove the gas and tidally strip the outskirts of the dark reveals a factor in common: they have all been satellites of another matter haloes of the now quenched seemingly-isolated UDGs, system in the past but are today central galaxies in the field. The which are born as star-forming field UDGs occupying dwarf-mass top panel of Fig. 3 shows an example of the orbit of one of our red dark matter haloes. Quiescent UDGs may therefore be found in UDGs. This dwarf interacted ∼ 4 billion years ago with a group that non-negligible numbers in filaments and voids, bearing the mark 13 has a virial mass M200(z = 0) ≈ 6.46 × 10 M but is found today of past interactions as stripped outer haloes devoid of dark matter ∼ 1.5 Mpc away, more than twice the virial radius of the group (virial and gas compared to dwarfs with similar stellar content. quantities refer to the radius enclosing 200 times the critical density of the Universe). The colour coding of the orbit, reflective of the Ultra-diffuse galaxies (UDGs) in groups and clusters are charac- dwarf colour at each time, shows that the reddening starts already as it terized by a puzzling wide range of dark matter and falls into the group and accelerates after the pericentric passage. The content 20–22, -like shapes 11, 23, old stellar populations 24 images of the simulated UDG (middle row) clearly show that its gas is and no substantial gas component. Their quiescence is not surprising removed as it approaches the pericenter, explaining its quiescence and given the high density environments they populate. On the other redness today in the field. The stellar size is not largely affected by the interaction, our red UDGs were all already extended before infall (see *E-mail: [email protected] Fig. 5 in the Supplementary Information).

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0.2 All simulated galaxies Simulated Central UDGs Isolated UDGs (Rong+20) DGSAT I (Martin-Navarro+19) 0.4 S82 DG1 (Roman+19) UDGs Coma (van Dokkum+15) UDGs Virgo (Lim+20)

7.4 7.6 7.8 8.0 8.2 8.4 8.6 8.8 9.0 log(M ) [M ]

Figure 1 – Definition of the UDG sample. Stellar mass versus size (M? vs rh?) relation for all simulated galaxies in the mass range log(M?/M ) = [7.4,9.1] in the TNG50 simulation (grey dots). Thin blue dashed curves indicate lines of constant surface brightness assuming a mass-to-light ratio equal to 1. The solid black line indicates the median size at fixed M? for the simulated galaxies. Yellow dashed curves show the 5th and 95th percentiles, with the shaded yellow region in between highlighting the sample of normal galaxies. Our sample of field UDGs (magenta stars) is defined as central galaxies with log(M?/M ) = [7.5,9] and stellar size above the 95th percentile (pink shaded region). Several observational data are shown in black edged symbols, where we transform 2D sizes Reff to 3D assuming rh? = 4/3Reff. Light blue diamonds indicate star-forming UDGs in low-density environments 14; the dark blue circle is the relatively isolated DGSAT I [27]; the red pentagon is UDG S82-DG-1, an isolated quiescent UDG 18. For comparison, we also show UDGs in the 28 (green crosses) and the 1 (pink x-symbols). Our UDG definition agrees well with observational samples, in particular for those in low density environments.

remaining cases being “pre-processing”, meaning that the UDG was first quenched in a moderate mass host which subsequently fell into a Objects in such external orbits, which are found far beyond the more massive system responsible for the energetic orbit. virial radius of their hosts, are known as backsplash galaxies 29, and are a natural consequence of the hierarchical assembly in ΛCDM. Our red UDGs are backsplash objects of systems in a wide range of A section of the simulated box with the location of red and blue 12 virial masses, including galaxy-sized haloes with M200 ≈ 2 × 10 M UDGs is illustrated in Fig. 3 (panel c), highlighting the red UDGs that to galaxy clusters, and are today on average at 2.1r200 from those are backsplash objects of galaxy-size haloes (green circles), located systems, or 1.7 ± 0.7 Mpc, but can reach as far as 3.35 Mpc in some mainly in low-density regions of the Universe. Red field UDGs cluster cases (see Fig. 7 in the Supplementary Information). In the large more than the blue ones, but they are all at substantial distances from majority of cases, (64.3%), the system responsible for the quenching their once-hosts. On average, the interactions occurred ∼ 5.5 Gyr ago and the launching beyond the virial radius is the same, with the and were moderately quick, with red UDGs spending typically 1.5 Gyr

2 1.0 ]

a 1

r 0 b All simulated galaxies y M

0.9 Central UDGs Star Forming [ 2 )

Central UDGs Quenched R

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r 0.5 g ( 0.4

0.3

0.2

0.1

7.4 7.6 7.8 8.0 8.2 8.4 8.6 8.8 9.0 log(M ) [M ]

Figure 2 – Dichotomy in colour and star formation rate of field UDGs. Panel a, colour (g - r) as a function of stellar mass for all simulated galaxies in this mass range (grey dots) and field UDGs (starred symbols). Most field UDGs are blue, but about a quarter of the sample populates the “red sequence”. These colours correlate with star formation rates (SFRs, small inset, panel b), where blue UDGs are star-forming and red −5 −1 UDGs are quiescent (SFR is zero in these objects but has been artificially shifted to 10 M yr for plotting purposes). Coloured symbols correspond to available observational data: isolated star-forming UDGs [14] with light blue diamonds and S82-DG-1 [18] with red pentagon.

(median) within the virial radius of the systems they are backsplash of a number of observational implications. First, the stellar populations (see Fig. 8 in the Supplementary Information). are old due to the quenching during the backsplash interaction. As shown in the left panel of Fig. 4, blue UDGs are comparatively Interestingly, there are extreme cases where the close pericenter younger, characterized by an extended star formation history as argued passage of the UDG results in its total ejection from the system in a in the case of observed field UDGs 15, and consistent with the overall way reminiscent of those in multiple-body interactions 30. Our most simulated population of field dwarfs (gray symbol). Note that the ages extreme UDG resides ∼ 3.35 Mpc away from its host and would appear inferred for isolated UDGs in observations are mostly in agreement as an extremely isolated object in a -like environment (see yellow with our blue UDG population. circle in bottom panel of Fig. 3). This UDG fell in as part of a galaxy- 13 size group into a group-size halo with M200(z = 0) = 3.36 × 10 M Second, the morphologies of red UDGs are always more spheroid- and was ejected more than 6 Gyr ago after its first pericenter (see dominated than their blue counterparts with similar stellar mass, which Fig. 9 in the Supplementary Information). might show spheroid or disc structure (see middle panel of Fig. 4), in agreement with previous work 31. Here, morphology is quantified by This scenario for the formation of quiescent UDGs in the field has the κrot parameter (Methods). We predict a shift towards early-type

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c 0 n a t log(M /M )=7.88 log(M /M )=8.21 log(M /M )=8.41 log(M /M )=8.4 log(M /M )=8.37 s i log(rh /kpc)=0.25 log(rh /kpc)=0.15 log(rh /kpc)=0.27 log(rh /kpc)=0.33 log(rh /kpc)=0.37 D 5 5 0 5 5 0 5 5 0 5 5 0 5 5 0 5 Distance [kpc]

50 c ] c p M

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Figure 3 – Formation of red UDGs in backsplash orbits and their location in the Universe. a, Example orbit of one of our quiescent UDG around its temporary (group-size) host halo, whose virial radius is indicated by the black line. The orbit is colour coded according to the instantaneous (g-r) colour in the dwarf (see colour bar in the top right) and shows that reddening starts right after pericenter. b, snapshot view of the stellar (red) and gas (blue) content of the UDG in different epochs along its orbit (times highlighted by the black squares in the upper panel). The gas is fully removed while approaching the pericenter, resulting in posterior aging and reddening of the stellar population. c, location of blue and red UDGs (starred symbols) in part of the simulation box. Structure is shown in the gray map as traced by all galaxies in the halo catalog. Red UDGs are spatially more clustered than blue ones, but some might exist even in very low density regions. For instance, the open green circles correspond to UDGs that are backsplash objects of galactic haloes with log(M200/M ) ≈ 12.5. The single yellow circle indicates the most extreme case of an ejected UDG located at ∼ 3.35 Mpc from its host.

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Figure 4 – Predicted properties of field red vs. blue UDGs. Panel a, average age of the population (mass-weighted) as a function of stellar mass. Red UDGs are older than the blue UDGs (average for red and blue are 5.21 ± 0.99Gyr and 9.40 ± 1.38Gyr, respectively). Blue UDGs have ages consistent with the overall sample of simulated normal (non-UDG) field dwarfs in the same mass range, as shown by the gray symbol with errorbars corresponding to 5th - 95th percentiles. Available observational data is shown for comparison (star-forming UDGs: [14] with light blue diamond; DGSAT I: [27] with yellow diamond and an average of UDGs in the Coma cluster 24 with the green diamond). Panel b, morphology quantified by the parameter κrot, where κrot > 0.6 (indicated by the vertical thin dashed line) highlights disk-dominated objects (see Methods). Red UDGs have typically more spheroidal morphology (lower κrot values) than the blue population at similar mass. Black thick dashed curve shows the median κrot at a given stellar mass. Panel c, virial mass (M200) - stellar mass relation. Red UDGs have stripped outer haloes and show today smaller virial masses at fixed stellar mass. The pink dashed curve corresponds to the abundance-matching model by [33]. Histograms, showing normalized numbers (N/Ntot) of UDGs, along all axes are included to ease the comparisons.

morphologies for red UDGs (low κrot) which is consistent with the large number of red field UDGs is found. Neutral gas and Hα studies picture where satellite galaxies are preferentially spheroid-dominated of red UDGs should also confirm a lack of gas in their interstellar due to transformations induced by the environment 32. medium.

Third and most important, backsplash galaxies have been stripped There are a few observational detections of quiescent UDGs in to some degree of their mass during the tidal interaction with their low density environments and they seem consistent with the picture past host system. While blue UDGs form in dwarf-mass haloes with emerging from our analysis. One of the first reported non-cluster 17 virial mass in the range log(M200/M ) = [10.3 − 11.2], red UDGs UDGs is DGSAT I [ ], which is located in the filament of the at the same stellar mass show smaller virial masses, with a median Pisces-Perseus . This is in excellent agreement with our log(M200/M ) = 9.73 (right panel in Fig. 4) due to this interaction. predictions, where most red field UDGs are nearby but outside groups 17 Red UDGs in the field should be clear outliers compared to predictions and clusters. DGSAT I lacks gas (as measured by Hα [ref. ]) and from abundance-matching models 33. The stripping occurs mostly in has a relatively old stellar population (8.1 ± 0.4 Gyr mass-weighted the outer halo, where the dark matter density profile of red UDGs falls age, 27) which is also within the range of properties predicted by our more steeply than the unperturbed blue UDG population (see Fig. 10 simulations. in the Supplementary Information). Unfortunately, the inner stellar velocity dispersion of red and blue UDGs – a possible observable – is Another interesting object is S82-DG-1, an extremely isolated statistically indistinguishable in our simulation. quenched UDG in a nearby void 18. Its isolation has been used to favour internal effects such as feedback to explain the possible origin A fourth implication in this scenario is that red UDGs are fully of UDGs, rather than due to high-density environmental effects. devoid of halo gas, which was all removed via ram pressure 34, 35 Here, we argue that S82-DG-1 fits the characteristic expected for along with the inner gas during the interaction with their hosts. We our simulated population of passive UDGs that were satellites of a have checked that no gas is re-accreted in these dwarfs, in contrast galactic-size host. S82-DG-1 is located at ∼ 55 kpc in projection 8 10 −1 with the gas mass Mgas = 10 to 10 M predicted in the haloes and at a redshift-distance less than ∆v = 145 km s from NGC 10 of blue UDGs in the field (this includes gas with distance (in kpc) 1211, a with stellar mass M? ≈ 1 × 10 M (see of 2rh? < r/kpc < r200, where rh? is the stellar half-mass radius). A Methods). Three of our simulated red UDGs have been backsplash 13 promising way to study the circumgalactic medium of these galaxies objects in galaxy-mass haloes M200 < 10 M and are found today down to very low column densities is to use background to ∼ 650 kpc from their hosts. Moreover, 12 red isolated UDGs (28.5%) 36 13 provide different absorption lines-of-sight across a halo . Although were quenched in galactic environments (M200 < 10 M ). Although this would be prohibitive on an individual UDG basis, a statistical the exact distance of S82-DG-1 to NGC 1211 is unknown, our detection (or lack of thereof) might be achieved once a sufficiently analysis provides support for the possible external nature of quenching

5 in S82-DG-1 induced by NGC 1211. The old stellar population compute colours) correspond to all stellar particles assigned to the inferred for S82-DG-1, 6 Gyr of age 18, is in excellent agreement galaxy (field SubhaloStellarPhotometrics in the halo catalog, 54). with the average time of the interactions found in our simulated sample. 55 The morphology parameter κrot is calculated following [ ] as We therefore propose backsplash orbits as a new mechanism to ex- follows. After rotating each galaxy to a reference frame where plain the presence of quiescent UDGs in low-density environments. the angular momentum of the stars (within rgal) points along the z This population of red and diffuse dwarfs results from the infall of nor- direction, we compare the energy in rotation around the z axis to the 2 mal star-forming field UDGs into galactic, group or cluster-size haloes total kinetic energy K as κrot = (1/K)Σ (1/2m jz /R), where jz is the responsible for stripping off their gas and propelling them to distances angular momentum of each stellar particle in the rotated system, m is ∼ 1 Mpc and beyond. In the most extreme cases, UDGs may even be their mass, R is their cylindrical radii and the sum is over stars within ejected several Mpc away from these systems. The predicted fraction of rgal. Defined in this way, the morphology parameter κrot has been red UDGs in our simulation in the studied mass range is ∼ 24%. Mild shown to correlate with other definitions of galaxy morphology 55, 56. clustering and old stellar populations, along with dark matter haloes Low κrot values correspond to spheroid-dominated objects, whereas of lower mass and the complete absence of gas in the galactic and cir- κrot > 0.6 is used to identify disk-dominated objects. cumgalactic region, are the expected telltales of this formation scenario for red UDGs. Future wide-field surveys targeting the surrounding of Galaxies are followed over time by means of Sublink merger trees groups and clusters may be the most promising way to uncover this 57. This allows us to track the mass, size and star formation histories population of elusive dwarfs predicted as a natural consequence of the of our sample over time. Note that the circumgalactic gas properties in assembly of haloes in ΛCDM. galactic-size and group-size haloes in TNG50 are in good agreement with observational constraints 26, 58 and may therefore provide a solid Methods theoretical ground to study environmental effects in our UDG sample. Simulation data and property determination. For our calcu- lations, we use the cosmological hydrodynamical TNG50 simulation The stellar mass for the lenticular galaxy NGC 1211 was estimated 25, 26 of the IllustrisTNG project 37–43. The IllustrisTNG galaxy from its V-band luminosity in [59] and assuming a mass to light ratio formation model presents improvements in physics due to the effects of 1 for simplicity. of active galactic nuclei and feedback 37, 44 compared to the original Illustris, its predecessor 45–47. The simulation was run using the Sample of field UDGs. The criterion to define UDGs varies moving-mesh code AREPO 48, 49. Besides gravity, the runs model a across different works in the literature. Here we define UDGs as the set of physical processes relevant to galaxy formation including gas most extended outliers of the stellar mass-size relation, following cooling and heating, star formation, metal enrichment, stellar and the philosophy introduced in [28]. The galaxy modeling used in the black hole feedback and magnetic fields. TNG100 and TNG50 simulation has been shown to agree well with observational constraints on the stellar mass-size relation of the galaxy The simulation is initialized at redshift z = 127 using the N-GENIC population 25, 60. In this work, we construct the mass-size relation code 50 with cosmological parameters consistent with the Planck mis- using well resolved galaxies, defined as those with dark matter mass 51 7 sion results : Ωm = ΩDM + Ωb = 0.3089, Ωb = 0.0486, cosmologi- mDM ≥ 5 × 10 M (with total dark matter mass assigned by SUBFIND −1 −1 6 cal constant ΩΛ = 0.6911 , Hubble constant H0 = 100hkms Mpc , to each subhalo), stellar mass m? ≥ 5 × 10 M and size rh? ≥ 0.3kpc; with h = 0.6774, σ8 = 0.8159 and spectral index ns = 0.9667. this results in a minimum number of ∼ 60 stellar and 110 dark matter particles. TNG50 is the smallest box from the TNG suite (50.7 Mpc (comoving) on a side compared to 100 Mpc and 300 Mpc) but the Fig. 1 in the main text shows the stellar mass-size relation, where 3 one with the highest resolution, a total of 2160 gas and dark matter the median at fixed M? is indicated by the thick black line. We notice particles are set in the initial conditions, resulting in a mass per particle that for log(M?/M ) < 7.5 the median starts to steadily increase 4 5 mbar = 8.4 × 10 M and mDM = 4.6 × 10 M for the baryons and towards lower-mass objects, an effect not found in observation and dark matter, respectively. The gravitational softening for the stars and of origin likely numerical. To be conservative, we study only dwarf dark matter is 0.29 kpc (comoving), whereas the gas has adaptive galaxies in the stellar mass range log(M?/M ) = [7.5,9]. More than softening down to 74 pc (physical). TNG50 is the only simulation 8600 dwarf galaxies in TNG50 satisfy this mass cut. The distribution of its kind and resolution that is able to follow such a wide range of of sizes at a given stellar mass is approximately log-normal. We, environments from dwarf haloes to clusters of galaxies. The TNG50 therefore, select the 5% most extended outliers at fixed M? as our box includes one (1) halo with log(M200/M ) > 14 and a substantial UDG population, deeming all galaxies within 5th- 95th percentiles number of less massive haloes (13 < log(M200/M ) < 14) that allows as ‘normal’ galaxies. This results in an average half mass radius the analysis of galaxies in the high-density environments of groups 2.5 ± 0.8kpc for our UDG population. From the UDGs identified in and clusters. this way, 176 are centrals to their haloes (or ‘field’ population) and constitute the sample analyzed here (the study of UDGs as satellites is The identification of groups is done via the friends-of-friends presented elsewhere by J.A.B. et al., manuscript in preparation). Fig. 1 (FoF) 52 algorithm followed by SUBFIND to identify substructure 53. shows that our definition for UDG galaxies is in very good agreement Subhaloes containing a stellar component are considered galaxies. with several observational samples of star-forming and quenched Galaxies are classified either as ‘centrals’ (main galaxy in each UDGs in low density environments 14, 18, 27. group) or ‘satellites’ otherwise. Here we use centrals to identify the population of dwarf galaxies in the field, meaning they are not Visualizations. Images shown in Fig. 3 were made using the satellites of any more massive system. Galaxy quantities such as Py-SPHViewer code61 v1.0.0. This code smooths the particle in- stellar mass M?, morphology, age and star formation rate are defined formation into two-dimensional histograms to reflect the underlying using particles within the ‘galactic radius’, which is defined as twice continuous density field. For the specific case of the small panels in the the half-mass radius of the stars: rgal = 2rh?. Luminosities (used to small panels in Fig. 3, we combined the information from the gas cells

6 (blue hues) and the stellar particles (red). Each stamp has 150x150 Additional information pixels and we use 12 neighbors for the smoothing. We use all gas or Supplementary information is available for this work. stellar particles identified to belong to this subhalo by SUBFIND which are within the image box (12 kpc (physical) on a side). For the bottom Supplementary Notes panel we use the XY coordinates of each subhalo in the halo catalog. The image is smoothed using a 3 neighbor kernel density estimation We collect here supplementary information to support our analysis and has 1000x500 pixels. presented in the Letter and Methods sections of this manuscript. The supplementary materials include 6 additional figures with their related discussion and references. Acknowledgements The authors would like to thank the referees for useful and con- Supplementary Information structive reports that helped improve this manuscript. JAB and Backsplash orbits may place galaxies well beyond the virial radius MGA acknowledge financial support from CONICET through PIP of their host halo, in some cases far enough that the subhalo finder (in 11220170100527CO grant. LVS is grateful for support from the NSF- this case SUBFIND) identifies them as isolated/central galaxies again. CAREER-1945310 and NASA ATP-80NSSC20K0566 grants. AP ac- Satellites in such orbits are needed to reproduce the observed frac- knowledge support from the Deutsche Forschungsgemeinschaft (DFG, tion of quiescent galaxies as a function of radius in galaxy groups and German Research Foundation) – Project-ID 138713538 – SFB 881 clusters out to at least twice the virial radii 29, 62–64. This backsplash (“The System”, subproject C09). DN acknowledges fund- mechanism has been found to be more efficient for low mass subhaloes ing from the Deutsche Forschungsgemeinschaft (DFG) through an 30, 65, therefore in the population. For instance, quiescent Emmy Noether Research Group (grant number NE 2441/1-1). FFM ac- dwarfs outside the virial radius of the Milky Way or Andromeda in the knowledges support through the program ”Rita Levi Montalcini” of the Local Group may be explained as backsplash objects 66. Italian MUR. MC is partially supported by NSF grants AST-1518257 Previous works linking the quenching of galaxies to backsplash or- and AST-1815475. PT acknowledges support from NSF grants AST- bits around groups and clusters have been analytical, semi-analytical 1909933, AST-200849 and NASA ATP grant 80NSSC20K0502. MV or based on N-body only simulations. Only recently, by using the acknowledges support through NASA ATP grants 16-ATP16-0167, 19- cosmological hydrodynamical simulation TNG50 it has been possible ATP19-0019, 19-ATP19-0020, 19-ATP19-0167, and NSF grants AST- to follow self-consistently the physical processes leading ultimately to 1814053, AST-1814259, AST-1909831 and AST-2007355. the gas removal and quenching of backsplash objects, for which high resolution is key in order to resolve not only the internal structure of Author contributions galaxies, but also the structure of the circumgalactic medium causing the gas stripping 67. The role of backsplashorbits on the quenching of The listed authors have made substancial contributions to this the dwarf population as a whole was studied in TNG5068, finding that manuscript, all coauthors read and commented on the document. JAB by excluding backsplash galaxies the fraction of quenched dwarfs in led the analysis of the simulation, compiled observational data from the the field with M ≥ 108 M is practically zero, in good agreement with literature and made all figures. LVS and MGA are responsible for the ? observational estimates from SDSS 69 and at most ∼ 5% at the lowest original idea and mentorship of JAB throughout the project. LVS led stellar mass considered here, M = 107.5 M . the writing of the manuscript and the response to the referee report with ? substantial contributions from JAB, MGA, AP, DN, FM and LH. Coau- Interestingly, the fraction of quiescent field UDGs in our sample thor MC provided the expertise on the observational consequences of is larger than taking the whole dwarf population: we find that 25% the results and on the study of quenching of dwarf galaxies. AP, DN, of field UDGs in the field are quenched when averaged in our whole FM, RP, PT, MV and LH are core members of the TNG50 simulation mass range, but up to 50% in the lowest mass half. For comparison, who set-up, developed and run the simulation this manuscript is based the quenched fraction for the normal dwarf population in the field is on. 7.8% in the whole mass range and 10.2% in the lowest mass half (see upper panel in Fig. 6). Restricting the analysis to only backsplash objects, 100% of backsplash UDGs are quenched while this fraction Data availability increases from 70% (for the low mass end) to 100% (high mass end This letter is based on snapshots, subhalo catalogs and merger explored here) in normal dwarfs. Although it is tempting to interpret trees from the cosmological hydrodynamical TNG50 simulation 25, 26 these numbers as UDGs being more susceptible to quenching during of the IllustrisTNG project 37–43. These data is publically available at their backsplash trajectories compared to normal dwarfs, we find that, https://www.tng-project.org/. Ascii tables with the simulation data for instead, the difference is due to a different distribution of host halo our sample of UDGs in Figs. 1, 2 and 4 are available in the public masses with which normal and UDGs galaxies have interacted in the repository: here. Source Data are provided with this paper. past.

For instance, for backsplash objects that interacted with Code Availability 13 M200 ≥ 10 M hosts, the fraction of quenched normal dwarfs Scripts used for reading/access to the snapshot, merger trees and and UDGs is, in both cases, > 90%. Normal dwarfs that interacted 13 subhalo data are publically available at the TNG database. Visual- with lower-mass hosts (M200 < 10 M ) are the ones showing a 50% izations were made using the publically available Py-SPHViewer code quenched fraction and driving down the overall quiescent fraction for 61. Any correspondence and/or request for materials pertaining to this the normal population. However, we have only 3 UDGs interacting manuscript should be directed to J.A.B. with galactic halos and therefore conclusive claims on whether the extended structure of UDGs may turn them more susceptible to ram-pressure stripping and other environmental effects will have to Competing interests wait until larger volume simulations of this kind provide a more solid The authors declare no competing interests. statistical basis for such a study.

7 ]

r 0 y

/ a M [ ) 2 R F S ( g o

l 4

) 2.5 b M / s

a 0.0 g M (

g 2.5 o l

] c c

p 0.5 k [ ) h

r 0.0 ( g o l 0.5 ] 8 d M [ ) 7 M (

g 6 o l

11 ] e

M 10 [ ) M

D 9 M ( g

o 8 l

0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 Time [Gyr]

Figure 5 – Time evolution of galaxy properties for quiescent and star-forming UDGs. From top to bottom: star formation rate (SFR, a), gas fraction defined as Mgas/M? ratio (b), stellar size (rh?, c), stellar mass (M?, d) and dark matter mass (MDM, e). Thin lines indicate individual galaxies while the thick continuous and dashed curves (red and blue respectively) correspond to medians of each UDG sample at a given time. The orange dotted vertical line correspond to the average time of infall for quiescent UDGs, while the shadowed yellow region indicates its standard deviation (see Fig. 8).

8 1.0 600 normal dwarf UDGs 0.8 400 0.6 N 0.4 200 0.2

0 0.0 frac. quench Dwarf in mass range 0.150 Isolated Star Forming Isolated Quenched

Isolated Quenched first infall 0.125

0.100

0.075

0.050

0.025

0.000 7.4 7.6 7.8 8.0 8.2 8.4 8.6 8.8 9.0 0.0 0.1 0.2 0.3 log(M ) [M ] N/NTot

Figure 6 – Formation mechanism of UDGs in the field. Halo spin for dwarf galaxies at a given stellar mass. Normal field dwarfs are shown in gray, while stars correspond to field star-forming UDGs (blue) and quiescent UDGs (red) measured at z = 0 and at infall (orange circles). The median spin at fixed M? of the normal dwarf population is indicated by the solid black curve and with error bars indicating 25th-75th percentiles, +0.017 the average value for all mass bins is λdwarf = 0.035−0.012. UDGs occupy preferentially higher-spin haloes. The median and rms dispersion for the UDGs are λblue = 0.059 ± 0.021 and λred,infall = 0.051 ± 0.024, for the blue and red population, respectively. Red UDGs are shown at infall (orange circles) and for comparison at z = 0 after the interaction with their hosts, which lowers their angular momentum via tidal stripping λred,z=0 = 0.017±0.011. The different halo spins in normal vs. UDG population is more clearly seen in the histograms on the right, with dashed lines indicating the medians of the normal, blue UDGs and red UDGs (at infall). The top panel shows, in addition to the stellar mass distribution of the dwarf population (gray histogram), the quenched fraction of field UDGs (red) and of field normal dwarfs (black line).

UDGs, star-forming (blue) and those quiescent (red). Individual This highlights that, when studying the tail of dwarfs with the galaxies are shown with thin lines, while medians are highlighted in most extended radii, the importance of backsplash orbits and of thick curves. The yellow shaded region indicates the average infall its associated quenching is enhanced with respect to the normal time of the red UDG population. The middle panel shows that red population of dwarfs. We emphasize that in the case of our quiescent UDGs sizes were already extended before their average infall time UDGs, backsplash orbits are responsible for quenching and placing and that the interaction had little impact afterwards. On average, our UDGs back in the field, but they play a non-dominant role on setting quenched UDG sample increased their size only by 25% compared to their extended sizes: quenched UDGs in the field were already diffuse their infall stellar half-mass radius. The top and second rows nicely and extended before infalling into their past hosts. illustrate that at the time of interaction with the hosts is when the removal of gas occurs followed by their quenching. This is in good We present this in more detail in Fig. 5, where we show several agreement with the galaxy transformations experienced by satellites 70–72 galaxy properties as a function of time for our population of field in the TNG simulation suite . The quenching experienced in

9 5 a b 8.4 8.4 3

4 8.2 8.2 ] ] ] 0 c 0 p 2 Group Halos Group Halos r Cluster Halos M M Cluster Halos M / Galactic Halos [ [ [ 2 Galactic Halos e ) ) e c 3 8.0 8.0 c n n M M a ( ( t a t g g s i s o o i l l d d 7.8 7.8 2 1

7.6 7.6 1 12.5 13.0 13.5 14.0 12.5 13.0 13.5 14.0 log(M200)[M ] log(M200)[M ]

Figure 7 – Location of backsplash UDGs in the field as a function of their past host halo virial mass. In panel a, present-day distance is shown in units of the virial radius of the past host halo, with a mean of ∼ 2.1 ± 0.6 × r200 for the sample. Panel b: same as before, but now distance is shown in Mpc, the dashed black line indicates the average virial radius (r200) at a given virial mass of the host. Encircled in black 12 are the UDGs that were “pre-processed” and quenched in haloes with an average M200 ∼ 9.16 × 10 M before falling into the last host halo they interacted with and that placed them on backsplash orbits. The colour code refers to stellar masses of each quenched UDGs at z = 0 and the shaded regions limit the type of halo they are backsplash galaxies of (galactic, groups or cluster haloes).

red UDGs around t ∼ 6 Gyr means that their early stellar growth should be systematically faster than their star-forming counterparts 73 12 (see d panel in Fig. 5), which continue to form stars until present day . 0.6 b a 0.4 % This still begs the question: what made quiescent UDGs be 0.2 10 so extended in the first place? We find that in TNG50, the UDG 0.0 0 5 10 population (red and blue) forms in dwarf haloes with biased-high t = t t [Gyr] out, r200 inf, r200 spins, and it is this excess of angular momentum compared to the 8 normal population what is responsible for their extended sizes (see Fig. 6, blue and orange symbols). Spins are calculated using the definition introduced in [74]. Note that quiescent UDGs should be

N considered at infall to study the origin of their sizes. At present-day, 6 their interactions with a past host has stripped off the outer halo layers lowering their initial angular momentum content (see red stars). A Kolmogorov-Smirnov test to compare the spin distribution of normal 4 (gray points, black curve) vs. UDGs (orange/blue symbols) retrieves a p-value = 2.63 × 10−7, confirming that λ values in both samples are not drawn from the same parent distribution. Large spins as 2 origin for the extended sizes in UDGs is in excellent agreement tinfall, r200 with one of the first theoretical models of UDG formation presented 2 t out, r200 and with recent kinematical modeling of observed UDGs in the field 75. 0 0 2 4 6 8 10 12 14 time [Gyr] Interestingly, contrary to this theoretical prediction, other simu- lations have found little dependence of their simulated field UDGs 3, 4 5 Figure 8 – Distribution of the infall and output times of field red on halo spin, citing instead burstiness and mergers as the origin UDGs. Panel a: infall (orange) and output (blue) times are defined for their extended sizes. For the high density environments of groups as the time each object crosses the virial radius (inwards or outwards and clusters, additional mechanisms have been proposed to alter 6–8, 10 respectively) of the host that placed them on backsplash orbits. Panel the size and the gas content of cluster UDGs . We present a b: distribution of time interval between infall and exit from the host. detailed study of the formation of UDGs in TNG50 and their relation On average these UDGs fell 5.83 ± 1.92 Gyr ago and stayed only ∼ 1.5 to environment in a companion paper (Benavides et al., in-prep). Gyr (median) within their temporary host haloes. Here, we focus instead on possible mechanisms to explain a specific sub-population of UDGs: those that are quiescent and in the field today.

10 4.0

5 0.2 0.1 0.4 0.7 (g r) [mag] 3.5

3.0 0 [kpc]

2.5

log(M , z0/M )=7.77 log(M /M )=12.27 5 200, inf 2.0 5 0 5 [kpc] 1.5

Distance [Mpc] tquench

1.0

tout 0.5 r 200 tinfall 0.0 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 time [Gyr]

Figure 9 – Example of our most extreme object: a quiescent UDG at 3.35 Mpc ejected from its host. The thick coloured line shows the distance of this specific UDG from its last group-sized host from which it is ejected at t ∼ 7.5 Gyr. As shown in Fig. 3 of the main text, the orbit is colour coded according to the instantaneous (g - r) galaxy colour (see colour bar). The black curve indicates the virial radius of the host group. 12 The UDG infalls into this group as part of an association of satellites in a galaxy-size group with M200 ∼ 10 M which is responsible for its quenching (“pre-processing”) ∼ 9 Gyr ago. Gray lines show the orbits of the other 17 satellites in the galactic group and magenta line shows the central of this group. The small snapshot view shows the UDG at present-day.

In Fig. 7 we show for each quiescent UDG the present-day dis- numbers. tance from the halo they backsplash from as a function of the present In order to reach large distances from their hosts, the interactions day virial mass of such haloes. The left panel shows that, on aver- placing UDGs on backsplash orbits must have occurred some time age, quenched UDGs are found at r ∼ 2.1r200 (in good agreement with ago. In Fig. 8 we show the distribution of infall times for all red UDGs 62 N-body only predictions ), but may reach as much as 4.9r200 (3.35 (orange), where infall is defined as the first time they crossed the virial Mpc). In the right panel, distances are shown in kpc. Dwarfs interact- radius of the FoF group they interacted with and is responsible for 13 ing with galactic haloes (M200 < 10 M ) may be found today ∼ 650 their placement on external orbits. The distribution is wide, but on kpc away from them. For groups and clusters, distances ≥ 1 Mpc are average red UDGs interacted with their hosts at ∼ 5.83 ± 1.92 Gyr ago common. Moreover, because of their foreign origin, the relative ve- (lookback time) or redshift z ∼ 0.6. The interaction in most cases is locities of backsplash UDGs with respect to their surrounding can be brief and corresponds to only a single pericentric passage. The blue significant. On average, quenched UDGs within 1 Mpc of their host to- histogram in Fig. 8 shows the distribution of departure times from the day have a relative velocity ∆v ∼ 200 km/s with respect to any other interaction group for each red UDG, tout, defined as the last time when surrounding galaxy (here we define the center of velocity by using they cross (outwards) the r200 of the host. The small inset panel shows 10 all galaxies with M? ≥ 10 M within 1 Mpc). The relative velocity that most UDGs spend less than 2 Gyr within their hosts (median 1.5 though increases with distance to their past hosts, reaching on average Gyr). ∆v ∼ 500 km/s (and individually up to ∼ 1000 km/s) if the red UDG is located today more than 1 Mpc away from its host. This is interesting The most extreme object in our sample is found today beyond in light of the observed quenched UDG S82-DG-1, which shows a high 3.35 Mpc (∼ 4.9 × r ) from the group it interacted with ∼ 6 Gyr 18 200 peculiar velocity . ago. Its unusual orbit, one in which the apocenter is larger than For most quiescent UDGs, the last system they interacted with – the turnaround radius, results from its common infall as part of a 12 and that placed them in backsplash orbits– is the same as that respon- galaxy-sized group (M200 = 1.86 × 10 M at t = 6 Gyr) hosting 6 sible for their star formation shutting off. However, a non-negligible multiple satellites itself (17 with M? > 5 × 10 M ). Fig. 9 shows the fraction (∼ 36%), are quenched as result of “pre-processing”, mecha- orbit of this specific quiescent UDG (thick coloured line) along with nism quantified in the TNG simulations76. We highlight such UDGs those of all galaxies in the galaxy-mass halo (gray lines for satellites, with a black circle in Fig. 7. The median virial mass of the halo where magenta for the central) that fall together into the final group host with 12 13 quenching in these galaxies happens is M200 = 9.16 × 10 M (mea- M200(z = 0) = 3.36 × 10 M . A snapshot view of the red UDG at sured at the time of quenching of each dwarf). This confirms that the present day is included. galaxy-mass haloes may also be capable of forming and hosting qui- escent UDGs, extending towards lower densities, beyond groups and The UDG is first quenched in the galaxy-mass system (“pre- clusters, the range of environments where red UDGs are found in large processing”) to later fall into the group and gain energy from

11 where tidal stripping operates, while both populations are indistin- 80 8 a guishable in the inner regions . This means that no major differences may be expected in mass (or velocity) estimates of star-forming and quiescent UDGs based on centrally concentrated tracers, such as stars 7 or inner globular clusters.

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